1. Field of the Invention
This invention relates to a semiconductor laser diode used as a source of light for an optical communication and, more particularly, to a semiconductor laser diode which has excellent temperature characteristics and low threshold current.
2. Description of the Prior Art
There have been a semiconductor laser diode of the type for a short wavelength region (.lambda.=0.78-0.89 .mu.m) and a semiconductor laser diode of the type for a long wavelength region (.lambda.=1.2-1.6 .mu.m). The temperature characteristics of a semiconductor laser diode of the type for the long wavelength region used mainly for an optical communication are deteriorated as compared with that for the short wavelength region. This causes drastic increase of threshold current and drastic decrease of differential quantum efficiency at higher temperature.
The threshold current of a laser diode is represented by an equation (1) as shown below, where operating threshold current I.sub.th is a function of a temperature T in Kelvin, and T.sub.0 denotes characteristic temperature in Kelvin. EQU T.sub.th =I.sub.th0 .multidot.exp (T/T.sub.0) (1)
As is understood from the above equation (1), a semiconductor laser diode having high characteristic temperature T.sub.0 is excellent due to its stable temperature characteristics.
In case of an AlGaAs (ternary crystalline solid solution) short wavelength band semiconductor laser diode, the characteristic temperature T.sub.0 is usually larger than 150 K.
On the other hand, in an InGaAsP (quaternary crystalline solid solution) long wavelength region semiconductor laser diode, T.sub.0 is as low as 60 K in the temperature range 0.degree. C.-40.degree. C., and it deteriorates further at higher temperature region.
Further, in case of a long wavelength region semiconductor laser diode having an active layer of a quantum well structure, T.sub.0 =150 K at lower temperature region than 40.degree. C., but T.sub.0 is as low as 60 K at higher temperature than 40.degree. C.
The dominant reason of the low characteristic temperature of the long wavelength region semiconductor laser diode is attributed to the carrier (electron) overflow from active layer to p-clad layer generated by Auger recombinations.
(Reference 1. Longwavelength Semiconductor Lasers, Agrawal and Outta, Van Nostrand Reinhold Company, N.Y. pp 71-141.)
In order to prevent such a carrier loss, "Double Carrier Confinement "(DCC)" hetero junction" has been proposed, and the following reference 2 is disclosed with respect to this.
Reference 2: IEEE J. Quantum Electron., Vol. QE-19, pp.1319-1327.
A representative DCC structure disclosed in the reference 2 will be described with reference to FIG. 5.
In case of a DCC structure shown in FIG. 6, an N-type InP clad layer 2, an InGaAsP first active layer 3, a p-type InP intermediate clad layer 4, a p-type InGaAsP second active layer 5, a p-type InP clad layer 6, a p-type InGaAsP contact layer 7, an n-type InP block layer 8 and are sequentially grown on an n-type InP substrate 1.
As is apparent from FIG. 7, in a long wavelength region semiconductor laser diode having such a DCC structure, its characteristic temperature T.sub.0 =130-210 K up to the vicinity of 80.degree. C. Therefore, the temperature characteristics are remarkably improved as compared with the conventional DH structure.
The reason for the improvement is considered as follows.
In the long wavelength region semiconductor laser diode, a material of InGaAsP/InP has larger generation of high energy hot electrons due to Auger recombinations as compared with that in GaAs/GaAlAs material system. Thus, as described above, hot electrons can overflow from active layer to p-type clad resulting lower characteristic temperature. On the contrary, in case of the DCC structure, the overflowed electrons are again trapped by the second active layer which contribute to lasing thereby improving the temperature characteristics.
The above-described conventional DCC structure has improved temperature characteristics. However, such DCC structure still has a problem which is that its threshold current density is still high.
The reason of such a problem is considered as follows. When an injection current is relatively low and carrier injection to the second active layer is small, the second active layer acts as an absorption layer of the light from the first active layer. The absorption of light causes the threshold current density to increase.